1. Field of the Invention
This invention relates to apparatus and methods for the disinfection of fluids and, in particular, to the disinfection of opaque industrial fluids with ultraviolet radiation.
2. Description of the Background
Coolant use in America's heavy industries has been a story of significant successes and failures. The efficacy of coolants in prolonging the life of various tools used in manufacturing has been high. This is due, in part, to the inclusion of specific organic and surfactant compounds that lubricate the materials and minimize oxidation damage and surface buildup of constituent substances on the tools. These compounds, as well as contaminating tramp oils that leak into the coolant from multiple sources, provide a very suitable nutrient base for microbial growth.
Coolants, as well as other industrial fluids, have traditionally possessed a fairly short useful life and need to be replenished often or even completely replaced. Although potentially toxic biocidal chemicals can be added to inhibit microbial growth, the biocidal effect eventually fails and supplementation with further biocides becomes impractical. Consequently, useful coolant life is only slightly extended. In addition, there are considerable environmental problems associated with disposal of used coolant, due in large part to the presence of these additives and other contaminants.
Over the past 50 years, machine coolant has been disposed of by dumping in drains, sewers and rivers, causing extensive and prolonged environmental ground pollution. In 1976, the EPA ruled that all oil-based coolants were contaminated waste and must be treated or a new way of disposal found (Public Law 94-580; Oct. 21, 1976). To accomplish this, centrifugation or filtration were considered as primary alternatives. Although filtration could remove some contaminant, filters often clogged or broke requiring more overall costs than would have been incurred by simply replacing the coolant. In addition, a successful filtration process only prolonged the life of the coolant by about two or three weeks making overall savings minimal. Centrifugation has been the principal mechanism for removing contaminated oils in larger machine tool plants. While centrifugation as an oil removal technique has a limited treatment rate, it has been used to reduce concentrations of contaminants, usually to about two percent. However, this partial removal does not prevent bacterial regrowth or breakdown of coolant and oil components.
Ultraviolet (UV) treatment has been used to disinfect clear waters and some wastewater as shown in U.S. Pat. Nos. 3,634,025; 3,700,406; 3,837,800; 3,889,123, 3,894,236; 4,471,225 and 4,602,162. Each of these U.S patents describes a method touted to be designed to sterilize water-based fluids. The principal idea behind this technique was that UV radiation would penetrate the clear liquid to kill offending microorganisms. The conventional technology of UV treatment is limited because total quartz systems have a tendency to foul easily and maintenance costs were high. UV treatment proved to be unsuccessful for industrial fluids such as coolants, as coolants are opaque, or substantially so, and often contain significant levels of contaminants such as hydraulic and way oils which are highly occlusive to ultraviolet light. Under these constraints, ultraviolet radiation cannot pass more than a very small distance, if at all, into the fluid stream (e.g. U.S. Pat. No. 3,456,107). These contaminants and coolants blocked UV transmission directly and also indirectly by adhering to wall surfaces of submerged quartz UV lamps or to the inner surfaces of the UV transmissible tubing in a dry system design, wherein UV lamps are kept separated from the fluid being treated.
A number of measures to prevent the degradation of coolant by microorganisms have been attempted with the objective to prolong the life of the coolant and to reduce odors and health risks associated with coolant spoilage. To minimize these risks and the hazards of contaminated coolant fluids, many facilities add appreciable levels of various biocides to coolant fluids to kill and inhibit the growth of microorganisms (e.g. U.S. Pat. No. 3,230,137). In general, coolants perform properly in the presence of these additives. However, allergic reactions to the biocides were common. In many cases, the biocides interacted with the skin of workers and caused various forms of hypersensitivity and dermatitis. In short, although bacterial counts can be reduced over the short term, biocides were often more problematic than the microorganisms themselves. Ultimately, the microorganisms overcome the biocides and the microbial degradation of coolant and contaminants results in foul odors in the work environment.
Conventional techniques, although useful in the short term, do not provide long term reduction of microbial counts in large industrial systems by more than a single log and, more importantly, only prolong coolant life for a short period despite their high cost. Other techniques such as aeration of coolant and thorough cleaning of the lines and machines through which the coolant flows proved to be largely unsuccessful in maintaining low levels of bacterial populations. Bacteria regrow in this environment due to the presence of available nutrients, and overcome inhibitory factors introduced by aeration or chemical management. Ultimately, the bacteria take hold throughout the coolant system.
Other, newer methods for the disinfection of coolant include pasteurization. In this process, coolant is heated to a pasteurizing temperature for a required period of time and subsequently cooled to an operating temperature. This process is energy intensive and the costs, resulting from the heating and cooling steps, are high. Although attempts have been made to keep pasteurization temperatures below the critical temperature that would destroy or denature the coolant, constant temperature cycling negatively effects coolant components. Consequently, there is a strong need for a safe and environmentally friendly method for the disinfection of industrial and other fluids.